The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit having layers containing an organic polymer called "kerogen," which upon heating decomposes to produce hydrocarbon liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil."
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen bearing shale and processing the shale above ground, or processing the oil shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972 to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by this reference. This patent describes in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by mining out a portion of the subterranean formation and then fragmenting a portion of the remaining formation to form a stationary, fragmented permeable mass of formation particles containing oil shale, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen containing retort inlet mixture into the retort as a gaseous combustion zone feed to advance the combustion zone through the retort. In the combustion zone oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the gaseous combustion zone feed into the combustion zone, the combustion zone is advanced through the retort. The combustion zone is maintained at a temperature lower than the fusion temperature of oil shale, which is about 2100.degree. F., to avoid plugging of the retort, and above about 1100.degree. F. for efficient recovery of hydrocarbon products from the oil shale.
The effluent gas from the combustion zone comprises combustion gas, carbon dioxide from mineral carbonate decomposition, and any gaseous portion of the combustion zone feed that does not take part in the combustion process. This effluent gas is essentially free of free oxygen and contains constituents such as oxides of carbon, water vapor, nitrogen, and sulfurous compounds. It passes through the fragmented mass in the retort on the advancing side of the combustion zone to heat oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and to a residue of solid carbonaceous material.
The liquid products and gaseous products are cooled by cooler particles in the fragmented mass in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, including shale oil, together with water produced in or added to the retort, are collected at the bottom of the retort and withdrawn to the surface through an access tunnel, drift or shaft. An effluent gas, referred to herein as off gas, containing combustion gas generated in the combustion zone, gaseous products including methane produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous portion of the combustion zone feed that does not take part in the combustion process is also withdrawn from the bottom of the retort.
Inorganic carbonates can be present in oil shale, notably carbonates of magnesium and calcium which decompose endothermically when heated to their decomposition temperatures. U.S. Pat. No. 4,036,299 to Cha, et al., assigned to the assignee of the present application and incorporated herein by this reference, describes a method of recovering shale oil from oil shale in an in situ oil shale retort in which a combustion zone is advanced through a fragmented permeable mass of formation particles containing oil shale and carbonates of magnesium and calcium. The patent discloses that the combustion zone is maintained at a temperature of from about 1100.degree. F. to about 1400.degree. F. (593.degree.-760.degree. C.), preferably from about 1200.degree. F. to about 1300.degree. F. (649.degree.-704.degree. C.), to obtain shale oil while avoiding excessive dilution of gaseous retorting products with carbon dioxide from decomposition of inorganic carbonates, notably calcium carbonate, in the oil shale.
Above-mentioned U.S. Pat. No. 3,661,423 to Garrett discloses briefly that mineral values can be leached from retorted oil shale in an in situ oil shale retort with water, acidic, or alkaline leaching agents. However, there is no description of the selective recovery of magnesium values from combusted oil shale in an in situ oil shale retort.
A number of patents have described the recovery of aluminum values from dawsonitic oil shale retorted above ground or by advancement of a combustion zone through an in situ oil shale retort, by leaching with aqueous leaching agents. Exemplary of these are U.S. Pat. Nos. 3,502,372 to Prats, 3,516,787 to Van Nordstrand, 3,572,838 to Templeton, 3,510,255 to Hall et al., and 3,642,433 to Dyni. The leaching agent typically is water or an alkaline aqueous solution, although the use of dilute acids has also been mentioned. The Van Nordstrand patent states that oil shale can contain from about 10 to 40 weight dolomite, and that dolomite in oil shale is decomposed upon retorting to form carbon dioxide, calcite, and magnesium oxide, the magnesium oxide tying up part of the silica in the oil shale to permit higher recovery of the aluminum values by leaching. Recovery of magnesium values it not disclosed in these patents.
The recovery of magnesium values from ground, calcined dolomite, a mineral form of calcium magnesium carbonate, is known. The selective leaching of magnesium values from dolomite calcined at 750.degree.-850.degree. C. with carbonated water has been described, for example, in U.S. Department of the Interior, Bureau of Mines Technical Paper 684, "The Bicarbonate Process for the Production of Magnesium Oxide," by H. A. Doerner et al (1946), the disclosure of which is incorporated herein by this reference. This paper describes the leaching of magnesium values from slurries of finely ground, calcined dolomite in well agitated mixing tanks.
The thermal decomposition of carbonates of magnesium and calcium in oil shale is described in E. J. Jukkola et al., "Thermal Decomposition Rates of Carbonates in Oil Shale," Industrial and Engineering Chemistry, 45 (1953), 2711-2714, which is incorporated herein by this reference. Data obtained by heating oil shale over a range of temperatures under various partial pressures of carbon dioxide are reported. Leaching of magnesium values from retorted oil shale is not described. A copy of the Jukkola, et al article accompanies this patent application.